Error when using cudnn7

[Naruto-Sasuke]

torch/install/share/lua/5.1/cudnn/SpatialConvolution.lua:42: attempt to perform arithmetic on field 'groups' (a nil value)
stack traceback:
        ...torch/install/share/lua/5.1/cudnn/SpatialConvolution.lua:42: in function 'resetWeightDescriptors'
        ...torch/install/share/lua/5.1/cudnn/SpatialConvolution.lua:93: in function 'checkInputChanged'
        ...torch/install/share/lua/5.1/cudnn/SpatialConvolution.lua:117: in function 'createIODescriptors'
        ...torch/install/share/lua/5.1/cudnn/SpatialConvolution.lua:177: in function 'func'

I wonder whether it is the 'cudnn_convert_custom.lua' that goes wrong.

[Naruto-Sasuke]

Have made a PR #78 .